This invention relates to a hot-melt adhesive, a hot-melt adhesive in the form of a sheet, a polyimide film having a layer or layers of the hot-melt adhesive on one side or both sides thereof, a substrate for flexible printed circuit boards obtained by adhering metal foils to a polyimide film via a hot-melt adhesive layer, and a substrate for metal-based printed circuit boards obtained by adhering metal foils to a metal plate via a hot-melt adhesive layer.
Substrates for flexible printed circuit boards have been produced by adhering metal foils to a polyimide film via an insulating adhesive layer. On the other hand, substrates for metal-based printed circuit boards which are excellent in heat dissipation have teen produced by adhering metal foils to a metal plate made of, for example, aluminum, steel, etc. via an insulating adhesive. In the field of electronics, materials having good high temperature properties and good workability are required. Further, as the insulating adhesive, there are required materials having good heat resistance, good adhesive strength and capable of adhering in a short time.
Epoxy resins, modified epoxy resins, phenolic resins and acrylic resins which have been used as such an adhesive have a defect in that they are poor in heat resistance.
On the other hand, polyimide resins are known as resins having excellent heat resistance. But since the polyimide resins do not dissolve and are not melted, they are not suitable for adhesives. When a polyamide acid which is a precursor of a polyimide is used, it should be used in the form of a solution obtained by dissolving it in a solvent, followed by removal of the solvent and imidazation. During the removal of the solvent and imidazation, voids are easily formed due to condensation water. Further since high temperature and long time are necessary for the imidazation, there are many limitations in practical use. Due to these problems, the polyimide resins are not used practically.